Advances in Colloid and Interface Science 145 (2009) 83–96

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Advances in Colloid and Interface Science

j ourna l homepage: www.e lsev ie r.com/ locate /c is

Silver nanoparticles: Green synthesis and their antimicrobial activities

Virender K. Sharma ⁎, Ria A. Yngard, Yekaterina LinChemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA

⁎ Corresponding author. Tel.: +1 321 674 7310; fax: +1E-mail address: vsharma@fit.edu (V.K. Sharma).

0001-8686/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.cis.2008.09.002

a b s t r a c t

a r t i c l e i n f o

Available online 17 September 2008

Keywords:

This review presents an ovethat have advantages over ctoxicity. Green synthetic

Silver colloid nanoparticlesEnvironmentally friendly synthesisIrradiationSilver-titanium dioxide nanoparticlesAntibacterial

methods include mixed-valence polyoxometallates, polysaccharide, Tollens,irradiation, and biological. The mixed-valence polyoxometallates method was carried out in water, anenvironmentally-friendly solvent. Solutions of AgNO3 containing glucose and starch in water gave starch-protected Ag NPs, which could be integrated into medical applications. Tollens process involves the reductionof Ag(NH3)2+ by saccharides forming Ag NP films with particle sizes from 50–200 nm, Ag hydrosols withparticles in the order of 20–50 nm, and Ag colloid particles of different shapes. The reduction of Ag(NH3)2+ byHTAB (n-hexadecyltrimethylammonium bromide) gave Ag NPs of different morphologies: cubes, triangles,

rview of silver nanoparticles (Ag NPs) preparation by green synthesis approachesonventional methods involving chemical agents associated with environmental

wires, and aligned wires. Ag NPs synthesis by irradiation of Ag+ ions does not involve a reducing agent and isan appealing procedure. Eco-friendly bio-organisms in plant extracts contain proteins, which act as bothreducing and capping agents forming stable and shape-controlled Ag NPs. The synthetic procedures ofpolymer-Ag and TiO2–Ag NPs are also given. Both Ag NPs and Ag NPs modified by surfactants or polymersshowed high antimicrobial activity against Gram-positive and Gram-negative bacteria. The mechanism of theAg NP bactericidal activity is discussed in terms of Ag NP interaction with the cell membranes of bacteria.Silver-containing filters are shown to have antibacterial properties in water and air purification. Finally,human and environmental implications of Ag NPs to the ecology of aquatic environment are brieflydiscussed.

© 2008 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 842. Silver nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843. Green synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

3.1. Polysaccharide method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 843.2. Tollens method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.3. Irradiation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 863.4. Biological method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.5. Polyoxometalates method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4. Ag NPs and their incorporation into other materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.1. Silver-doped hydroxyapatite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 884.2. Polymer-silver nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.2.1. Poly(vinyl alcohol)-silver nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 884.3. Silver nanoparticles on TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5. Antimicrobial activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.1. Studies: mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.2. The battle against infection: Ag NPs and their incorporation into the medical field . . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.2.1. Ag NPs and HIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935.3. Antibacterial water filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 935.4. Antimicrobial air filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

321 674 8951.

l rights reserved.

84 V.K. Sharma et al. / Advances in Colloid and Interface Science 145 (2009) 83–96

6. Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 936.1. Human Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 936.2. Environmental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

7. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

1. Introduction

The application of nanoscale materials and structures, usuallyranging from 1 to 100 nanometers (nm), is an emerging area ofnanoscience and nanotechnology. Nanomaterials may provide solu-tions to technological and environmental challenges in the areas ofsolar energy conversion, catalysis, medicine, and water treatment[1,2]. This increasing demand must be accompanied by “green”synthesis methods. In the global efforts to reduce generatedhazardous waste, “green” chemistry and chemical processes areprogressively integrating with modern developments in science andindustry. Implementation of these sustainable processes should adoptthe 12 fundamental principles of green chemistry [3–7]. Theseprinciples are geared to guide in minimizing the use of unsafeproducts and maximizing the efficiency of chemical processes. Hence,any synthetic route or chemical process should address theseprinciples by using environmentally benign solvents and nontoxicchemicals [3].

Nanomaterials often show unique and considerably changedphysical, chemical and biological properties compared to theirmacro scaled counterparts [8]. Synthesis of noble metal nanoparticlesfor applications such as catalysis, electronics, optics, environmental,and biotechnology is an area of constant interest [9–15]. Gold, silver,and copper have been used mostly for the synthesis of stabledispersions of nanoparticles, which are useful in areas such asphotography, catalysis, biological labeling, photonics, optoelectronicsand surface-enhanced Raman scattering (SERS) detection [16,17].Additionally, metal nanoparticles have a surface plasmon resonanceabsorption in the UV–Visible region. The surface plasmon band arisesfrom the coherent existence of free electrons in the conduction banddue to the small particle size [18,19]. The band shift is dependent onthe particle size, chemical surrounding, adsorbed species on thesurface, and dielectric constant [20]. A unique characteristic of thesesynthesized metal particles is that a change in the absorbance orwavelength gives a measure of the particle size, shape, andinterparticle properties [20,21]. Moreover, functionalized, biocompa-tible and inert nanomaterials have potential applications in cancerdiagnosis and therapy [22–26]. The target delivery of anticancer drugshas been done using nanomaterials [22]. With the use of fluorescentand magnetic nanocrystals, the detection and monitoring of tumorbiomakers have been demonstrated [24,25].

Generally, metal nanoparticles can be prepared and stabilized byphysical and chemical methods; the chemical approach, such aschemical reduction, electrochemical techniques, and photochemicalreduction is most widely used [27,28]. Studies have shown that thesize, morphology, stability and properties (chemical and physical) ofthe metal nanoparticles are strongly influenced by the experimentalconditions, the kinetics of interaction of metal ions with reducingagents, and adsorption processes of stabilizing agent with metalnanoparticles [21,22]. Hence, the design of a synthesis method inwhich the size, morphology, stability and properties are controlled hasbecome a major field of interest [29].

2. Silver nanoparticles

Silver is widely known as a catalyst for the oxidation of methanolto formaldehyde and ethylene to ethylene oxide [30]. In the UnitedStates, more than 4×106 tons of silver were consumed in 2000.

Colloidal silver is of particular interest because of distinctive proper-ties, such as good conductivity, chemical stability, catalytic andantibacterial activity [31]. For example, silver colloids are usefulsubstrates for surface enhanced spectroscopy (SERS), since it partlyrequires an electrically conducting surface [19,32,33]. Also, theexposure of silver ions to light reduces them into 3–5 atoms clustersof silver, which catalyzes a gain of ∼108 atoms in latent image to bevisible [34].

Chemical reduction is the most frequently applied method for thepreparation of silver nanoparticles (Ag NPs) as stable, colloidaldispersions in water or organic solvents [35,36]. Commonly usedreductants are borohydride, citrate, ascorbate, and elemental hydro-gen [37–45]. The reduction of silver ions (Ag+) in aqueous solutiongenerally yields colloidal silver with particle diameters of sever-al nanometers [36]. Initially, the reduction of various complexes withAg+ ions leads to the formation of silver atoms (Ag0), which is followedby agglomeration into oligomeric clusters [46]. These clusterseventually lead to the formation of colloidal Ag particles [46]. Whenthe colloidal particles are much smaller than the wavelength of visiblelight, the solutions have a yellow color with an intense band in the380–400 nm range and other less intense or smaller bands at longerwavelength in the absorption spectrum [19,32,33]. This band isattributed to collective excitation of the electron gas in the particles,with a periodic change in electron density at the surface (surfaceplasmon absorption) [47–49].

Previous studies showed that use of a strong reductant such asborohydride, resulted in small particles that were somewhat mono-disperse, but the generation of larger particles was difficult to control[50,51]. Use of a weaker reductant such as citrate, resulted in a slowerreduction rate, but the size distribution was far from narrow[37,38,52]. Controlled synthesis of Ag NPs is based on a two-stepreduction process [51]. In this technique a strong reducing agent isused to produce small Ag particles, which are enlarged in a secondarystep by further reductionwith a weaker reducing agent [37]. Differentstudies reported the enlargement of particles in the secondary stepfrom about 20–45 nm to 120–170 nm [53–55]. Moreover, the initial solwas not reproducible and specialized equipment was needed [39]. Thesyntheses of nanoparticles by chemical reduction methods aretherefore often performed in the presence of stabilizers in order toprevent unwanted agglomeration of the colloids.

The green synthesis of Ag NPs involves three main steps, whichmust be evaluated based on green chemistry perspectives, including(1) selection of solvent medium, (2) selection of environmentallybenign reducing agent, and (3) selection of nontoxic substances for theAg NPs stability [7]. Based on this approach, we have reviewed thegreen-chemistry type Ag NP synthesis processes. The synthesis ofpolymer-Ag NPs and Ag NPS on TiO2 are also summarized because oftheir industrial and environmental importance. Finally, antimicrobialactivities of Ag NPs with some examples of mechanism are presented.Implications of Ag NPs to human health and environment are brieflydiscussed.

3. Green synthesis

3.1. Polysaccharide method

In this method, Ag NPs are prepared using water as anenvironmentally benign solvent and polysaccharides as a capping

Fig. 2. Concentration of free Ag+ ions and silver–ammonia complexes versus theammonia concentration. The total [silver]=0.001 M.

85V.K. Sharma et al. / Advances in Colloid and Interface Science 145 (2009) 83–96

agent, or in some cases polysaccharides serve as both a reducing and acapping agent. For instance, synthesis of starch-Ag NPs was carriedout with starch as a capping agent and β-D-glucose as a reducing agentin a gently heated system [7]. The starch in the solutionmixture avoidsuse of relatively toxic organic solvents [56]. Additionally, the bindinginteractions between starch and Ag NPs are weak and canbe reversible at higher temperatures, allowing separation of thesynthesized particles.

In a case of dual polysaccharide function, AgNPswere synthesized bythe reduction of Ag+ inside of nanoscopic starch templates, Fig. 1. Theextensive network of hydrogen bonds in the templates provides surfacepassivation or protection against nanoparticle aggregation [7,57]. Also,Ag NPs were synthesized by using negatively charged heparin as areducing/stabilizing agent byheating a solution of AgNO3 andheparin to70 °C for ∼8 h [58]. TEM images of these Ag NPs revealed an increase inparticle size with increased concentrations of both, AgNO3 and heparin[58]. Furthermore, changes in heparin concentration varied Ag NP sizeand morphology suggesting that heparin must behave as a nucleationcontroller and stabilizer [58]. The AgNPswere highly stable and showedno signs of aggregation after two months [58].

In another study, stable Ag NPs (10–34 nm) were synthesized byautoclaving a solution of AgNO3 and starch (capping/reducing agent)at 15 psi and 121 °C for 5 min [59]. The Ag NPs were stable in solutionfor three months at ∼25 °C. Smaller Ag NPs (≤10 nm) were producedby mixing two solutions of AgNO3 containing starch, a capping agent,and NaOH solutions containing glucose, a reducing agent, in aspinning disk reactor with a reaction time of less than 10 min [60].Importantly, starch-protected nanoparticles can be easily integratedinto systems for biological and pharmaceutical applications.

3.2. Tollens method

The Tollens synthesismethod gives Ag NPswith a controlled size ina one-step process [61–64]. The basic Tollens reaction involves thereduction of Ag(NH3)2+(aq), a Tollens reagent, by an aldehyde, Eq. (1).

AgðNH3Þþ2 ðaqÞ þ RCHOðaqÞ→AgðsÞ þ RCOOHðaqÞ ð1Þ

In the modified Tollens procedure, Ag+ ions are reduced bysaccharides in the presence of ammonia, yielding Ag NP films withparticle sizes from 50–200 nm, Ag hydrosols with particles in theorder of 20–50 nm, and Ag NPs of different shapes [62,63]. Ag(NH3)2+ isa stable complex ion resulting from ammonia's strong affinity for Ag+,therefore the ammonia concentration and nature of the reductantmust play a major role in controlling the Ag NP size [63].

To better understand the synthesis process lets consider thisexample. A research study on the saccharide reduction of Ag+ ions bythe modified Tollens process revealed that the smallest particles wereformed at the lowest ammonia concentration [63]. Specifically,

Fig. 1. Typical TEM image of starch silver nanoparticles. The scale bar corresponds to20 nm (reproduced from [7] with permission from the American Chemical Society).

glucose and the lowest ammonia concentration, 0.005 M, resulted inthe smallest average particle size of 57 nmwith an intense maximumof the surface plasmon absorbance at 420 nm. Furthermore, asimultaneous increase in particle size and polydispersity was detectedwith an increase in [NH3] from 0.005 M to 0.2 M [63].

To gain further insight on the effect of ammonia, it is important toknow the chemical speciation of Ag(I) in the studied system. Both, Ag

Fig. 3. Log-normal size distribution of silver nanoparticles at different pH for glucoseandmaltose in 0.005M ammonia concentration (reproduced from [66] with permissionfrom the American Chemical Society).

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(NH3)+ and Ag(NH3)2+ are produced in the reaction solution as shownin Eqs. (2) and (3), where the formation constants are log β1=3.367and log β2=7.251, respectively [65].

Agþ þ NH3⇆AgðNH3Þþ logβ1 ¼ 3:367 ð2Þ

Agþ þ 2NH3⇆AgðNH3Þþ2 logβ2 ¼ 7:251 ð3Þ

The concentrations of the possible Ag species using formationconstants expressed in Eqs. (2) and (3) as a function of [NH3] aredisplayed in Fig. 2. A decrease in [Ag+] in the presence of NH3 results ina decrease in the reduction rate to Ag(s), Eq. (1), and thus is reflectedin the particle size. Initially this would lead to a decrease in theformation of stable Ag nuclei. In the latter stage of particle growth, thelimited presence of nuclei would lead to larger particles.

Likewise, Ag NPs of controllable sizes were synthesized byreduction of [Ag(NH3)2]+ with two monosaccharides (glucose andgalactose) and two disaccharides (maltose and lactose) [66]. Thesynthesis was carried out at various ammonia concentrations (0.005–0.20 M) and pH conditions (11.5–13.0) resulting in average particlesizes of 25–450 nm. As anticipated, the average particle size increasedwith increasing [NH3]. A maximum particle size was reached at theconcentration of 0.035 M for disaccharides and 0.20 M for mono-saccharides. The difference in structure of monosaccharides anddisaccharides influences the particle size with disaccharides giving onaverage smaller particles than monosaccharides at pH 11.5 (e.g. Fig. 3).Furthermore, particles obtained at pH 11.5 were smaller than those atpH 12.5. Polydispersity also decreased by lowering the pH (Fig. 3).Maltose gave Ag NPs with the most narrow size distribution and thesmallest average size of 25 nm. To extend shelf life, Ag NPs were

Fig. 4. TEM images of silver nanoparticles: (a) cubes; (b) triangles; (c) wires; (d) an alignmen

stabilized by two surfactants, sodium dodecyl sulfate-SDS andpolyoxyethylenesorbitane monooleate-Tween 80, and a polymer,polyvinylpyrrolidone-PVP 360 [67,68].

A modified Ag mirror reaction (Tollens reaction) is an example of asynthesis route yielding Ag NPs of different shapes. Ag NPs of variousmorphologies with b10 nm diameters were synthesized in water byadjusting the concentrations of n-hexadecyltrimethylammoniumbromide (HTAB) and the Tollens reagent, Ag(NH3)2+, at 120 °C[69,70]. TEM images of Ag NPs obtained by this method are shownin Fig. 4.

3.3. Irradiation method

Ag NPs can be successfully synthesized by using a variety ofirradiation methods. For example, laser irradiation of an aqueoussolution of Ag salt and surfactant can fabricate Ag NPs with a well-defined shape and size distribution [71]. No reducing agent is requiredin this method. Additionally, laser was applied in a photo-sensitizationtechnique for the synthesis of Ag NPs using benzophenone [72]. Here,low laser powers at short irradiation times gave Ag NPs of ∼20 nm,while an increased irradiation power gave nanoparticles of ∼5 nm.The formation of Ag NPs by this photo-sensitization technique wasalso achieved using amercury lamp [72]. In the visible light irradiationstudies, photo-sensitized growth of Ag NPs using thiophene as asensitizing dye [73] and Ag NP production by illumination of Ag(NH3)+

in ethanol has been carried out [74].Synthesis procedures using microwave irradiation has also been

employed. Microwave radiation of a carboxymethyl cellulose sodiumand silver nitrates solution produced uniform Ag NPs that were stablefor two months at room temperature [75]. The microwave irradiation

t of wires. (reproduced from [70] with permission from the American Chemical Society).

87V.K. Sharma et al. / Advances in Colloid and Interface Science 145 (2009) 83–96

of a AgNO3-ethylene-glycol-H2[PtCl6]-poly(vinylpyrrolidone) solutiongave Ag NPs of different shapes within 3 min. [76]. Recently, the use ofmicrowave radiation to synthesize nearly monodisperse Ag NPs usingbasic amino acids as reducing agents and soluble starch as a protectingagent has been shown [77].

Ionizing radiation can reduce Ag+ ions in Ag NPs synthesis [78–83].In one study, Ag NPs of N10 nmwere produced in supercritical ethaneat 80 °C and 80–120 bar with methanol as a solvent [78]. The solvatedelectrons reduced the Ag+ ions and a characteristic plasmon absorp-tion was detected within 1–10 s after the ionization pulse.

Furthermore, radiolysis has been applied in the AgNP production. Theradiolysis of Ag+ ions in ethylene glycol was studied [82]. Here, theformation of Agowas observed at 350 nm (k (Ag++e−solv)=2.8×109M−1 s−1)and the surface plasmon band appeared slowly at 400 nm with acoalescence cascade k=2×106 M−1 s−1 [82]. Furthermore, Ag NPssupported on silica aerogel were synthesized using gamma radiolysis[83]. The Ag clusters were stable in the 2–9 pH range and startedagglomeration at pH N9 [83]. In another work, oligochitosan as astabilizer was used in preparation of Ag NPs by gamma radiationsynthesizing 5–15 nm stable Ag NPs in a 1.8–9.0 pH range [79].Gamma radiation in acetic water solution containing AgNO3 andchitosan gave particles with an average diameter of 4–5 nm [80]. AgNPs of different size (60–200 nm) have also been synthesized byirradiatinga solution, preparedbymixingAgNO3 andpoly-vinyl-alcohol,with 6 MeV electrons [84]. The variation of electron fluencefrom 2×1013–3×1015 e cm−2 produced Ag NPs of narrow size distribu-tion (60–10 nm) [84].

The pulse radiolysis technique has been applied to study thereactions of inorganic and organic species in Ag NP synthesis [85–87].This technique was successfully applied to understand the factorscontrolling the shape and size of Ag NPs produced by a commonreduction method using citrate ions [88]. Interestingly, the citrate ionfunctioned as a reductant, a complexant, and a stabilizer. Recently, apulse radiolysis study was performed to demonstrate the role ofphenol derivatives in the formation of Ag NPs by the reduction of Ag+

ions with dihydroxybenzene [89].In a morphology conversion study, suspensions of Ag nanospheres

were converted to triangular Ag nanocrystals, so called nanoprisms, inhigh yield using photoinduced electron transfer [90]. This photo-induced method was extended to demonstrate synthesis of relativelymonodisperse nanoprisms with desired edge lengths of 30–120 nm[91]. With the use of dual-beam illumination, the nanoparticle growthprocess could be controlled.

3.4. Biological method

Extracts from bio-organisms may act both as reducing and cappingagents in Ag NPs synthesis. The reduction of Ag+ ions by combinationsof biomolecules found in these extracts such as enzymes/proteins,amino acids, polysaccharides, and vitamins [92,93] is environmentallybenign, yet chemically complex. An extensive volume of literaturereports successful Ag NP synthesis using bioorganic compounds.

For example, the extract of unicellular green algae Chlorella vulgariswas used to synthesize single-crystalline Ag nanoplates at roomtemperature [94]. Proteins in the extract provide dual function of Ag+

reduction and shape-control in the nanosilver synthesis. The carboxylgroups in aspartic and/or glutamine residues and the hydroxyl groups intyrosine residues of the proteins were suggested to be responsible forthe Ag+ ion reduction [94]. Carrying out the reduction process by asimple bifunctional tripeptide Asp-Asp-Tyr-OMe further identified theinvolvement of these residues. This synthesis process gave small Agnanoplates with low polydispersity in good yield (N55%) [94].

Plant extracts from live alfalfa, the broths of lemongrass, geraniumleaves and others have served as green reactants in Ag NP synthesis[95–97]. The reaction of aqueous AgNO3 with an aqueous extract ofleaves of a common ornamental geranium plant, Pelargonium grave-

olens, gave Ag NPs after 24 h [96]. The reaction timewas reduced to 2 hby heating the reaction mixture just below the boiling point [98].Secreted proteins in spent mushroom substrate reduced Ag+ to giveuniformly distributed Ag-protein (core–shell) NPs with an averagesize of 30.5 nm [99]. A vegetable, Capsicum annuum L., was used to alsosynthesize Ag NPs [100].

Studying the synthesis of Ag NPs with isolated/purified bioor-ganics may give better insight into the system mechanism.Glutathione (γ-Glu-Cys-Gly-) as a reducing/capping agent canproduce water-soluble and size tunable Ag NPs that easily bind tomodel protein (bovine serum albumin) — attractive for medicalapplications [101]. Tryptophan residues of synthetic oligopeptides atthe C-terminus were identified as reducing agents giving Ag NPs[102]. Furthermore, Ag NPs were successfully synthesized byVitamin E in the Langmuir–Blodgett technique, by biosurfactants,such as sophorolipids, [103–105] and by L-Valine-based oligopep-tides with chemical structures, Z-(L-Val)3-OMe and Z-(L-Val)2-L-Cys(S-Bzl)-OMe [106]. The sulfur content in the Z-(L-Val)2-L-Cys(S-Bzl)-OMe controls the shape and size of Ag NPs, which suggests theinteraction between the Ag+ ion and the thioether moiety of thepeptide [106]. Oleic acid has also been used in environmentally-friendly synthesis of organic-soluble Ag NPs [107].

Several microorganisms have been utilized to grow Ag NPsintracellularly or extracellularly [108–114]. For instance, Ag containingnanocrystals of different compositions were synthesized by Pseudo-monas stutzeri AG259 bacterium [108]. In Fusarium oxysporum fungus,the reduction of Ag+ ions was attributed to an enzymatic processinvolving NADH-dependent reductase [113]. The white rot fungus,Phanerochaete chrysosporium, also reduced Ag+ ion to form Ag NPs; aprotein was suggested to cause the reduction [114]. Possible involve-ment of proteins in synthesizing Ag NPs was observed in filamentouscyanobacterium, Plectonema boryanum UTEX 485 [115]. Moreover, Ag+

reduction by culture supernatants of Klebsiella pneumonia, Escherichiacoli (E. coli), and Enterobacter cloacae (Enterobaceteriacae) producedrapid formations of Ag NPs [116].

3.5. Polyoxometalates method

Polyoxometalates, POMs, have the potential of synthesizing Ag NPsbecause they are soluble inwater and have the capability of undergoingstepwise, multielectron redox reactions without disturbing theirstructure [117–119]. For example, Ag NPs were synthesized byilluminating a deaerated solution of POM/S/Ag+ (POM: [PW12O40]3−,[SiW12O40]4−; S:prpan-2-ol or 2,4-dichlorophenol) [119]. In this methodPOMs serve both as a photocatalyst, a reducing agent, and as a stabilizer[119]. In another study, one-step synthesis and stabilization of Agnanostructures with MoV–MoVI mixed-valence POMs in water at roomtemperature has been demonstrated [120]. This method did not use acatalyst or a selective etching agent.

Ag NPs of different shape and size can be obtained using differentPOMs in which the POMs serve as a reductant and a stabilizer.For instance, a salt, Ag2SO4, and POMs, (NH4)10[MoV)4(MoVI)2O14

(O3PCH2PO3)2(HO3PCH2PO3)2]-15 H2O and H7[β-P(MoVI)4(MoVI)8O40],were reacted. After several minutes of mixing a characteristic SPR bandat 400 nm for Ag NPs appeared and the location of the peak was notsignificantly affected by the initial concentration of Ag2SO4 (Fig. 5a)[120]. The Ag NPs obtained were spherical and quasi-monodispersedwith a diameter of ∼38 nm (Fig. 5b), the particle size distribution wasquantitatively displayed in a histogram (Fig. 5c). The single Ag NP inFig. 5d has a Ag-POM core-shell structurewith a∼2 nm thick POM layer.

4. Ag NPs and their incorporation into other materials

The unique properties of Ag NPs have been extended into a broaderrange of applications. Incorporation of Ag NPswith othermaterials is anattractive method of increasing compatibility for specific applications.

Fig. 5. (a) SPR spectra of Ag nanoparticles obtained from different molar ratios, (b) a representative TEM image of Ag nanoparticles obtained from the mixture with γ) 4, (c) sizehistogram of Ag nanoparticles of about 200 NPs counted from TEM image showing the distribution of Ag NPs, and (d) a magnified Ag nanoparticle. (reproduced from [70] withpermission from the American Chemical Society).

88 V.K. Sharma et al. / Advances in Colloid and Interface Science 145 (2009) 83–96

4.1. Silver-doped hydroxyapatite

There is interest in inorganic-inorganic hybrid nanocompositesmaterials because of their industrial and medical applications [121–124]. Recently, one-step synthesis of anisotropic Ag nanocrystals wasachieved by reducing aqueous Ag+ ion by the electron transfer fromthe surface of hydroxyapatite (HA) [125]. The hydroxyl group in thisprocess acted both as a reducing and a binding agent to give highlyoriented flat rod and needle-like Ag NPs [125]. A microwave processwas also applied to synthesize nanosize Ag-substituted HA with alength of 60–70 nm and width of 15-20 nm [126].

4.2. Polymer-silver nanoparticles

Nanocomposite materials consisting of metallic nanoparticlesincorporated in or with polymers have attracted much attentionbecause of their distinct optical, electrical and catalytic properties,which have potential applications in the fields of catalysis, bioengi-neering, photonics, and electronics [47,127–130]. Polymers areconsidered a good host material for metal nanoparticles as well asother stabilizing agents such as citrates, organic solvents (THF or THF/MeOH), long chain alcohols, surfactants, and organometallics[9,131,132]. The organic solvents are though not as environmentalbenign.

Different chemical and physical methods exist to prepare metal-polymer composites [46,133,134–142]. A successful preparation ofnanoparticles is determined by the ability to produce particles withuniform distributions and long stability, given their tendency torapidly agglomerate in aqueous solution [130,133]. The main fabrica-tion approach is to disperse previously prepared particles in thepolymer matrix [141,142]. This method is often referred to as the

evaporation method since the polymer solvent is evaporated from thereaction mixture after NP dispersion. However, this often leads toinhomogeneous distribution of the particles in the polymer. Onesolution is the in situ synthesis of metal particles in the polymermatrix, which involves the dissolution and reduction of metal salts orcomplexes into the matrix [138,143]. Or, another approach is a systemin which simultaneous polymerization and metal reduction occur.

For example, the in situ reduction of Ag+ ions in poly(N-vinyl-2-pyrrolidone) (PVP) by microwave irradiation produced particles withnarrow size distribution [144] and Ag NPs incorporated in acacia, anatural polymer, had been made under mild condition [145]. Or, aconventional heating method to polymerize acrylonitrile simulta-neously reduces Ag+ ions resulting in homogeneous dispersal andnarrow size distributions of the Ag NPs in the silver-polyacrylonitrile(Ag-PAN) composite powders [138]. Further, size-controlled synthesisof a Ag nanocomplex was recently achieved in the reduction of AgNO3

by a UV-irradiated argine-tungstonsilicate acid solution [146]. Othervarious metal-polymer nanocomposites have been prepared by thesereduction methods, such as poly(vinyl alcohol)-Ag, Ag-polyacryla-mide, Ag-acrylonitrile (Ag-PAN), Ag2Se-polyvinyl alcohol, Ag-poly-imide, Au-polyaniline, and Cu-poly(acrylic acid) [143,147]. Due to itsgrowing importance in a multitude of industries, let's explore poly(vinyl alcohol)-Ag synthesis and applications in more detail.

4.2.1. Poly(vinyl alcohol)-silver nanoparticlesPoly(vinyl alcohol) (PVA) is a biologically friendly polymer since it

is water soluble and has extremely low cytotoxicity [148]. This allowsa wide range of potential biomedical applications. It is frequently usedas a stabilizer due to its optical clarity, which enables investigation ofthe nanoparticle formation [149,150]. PVA is classified into grades ofpartially (85–89%) and fully (97–99.5%) hydrolyzed polymers (Fig. 6).

Fig. 7. The surface-plasmon absorbance spectrum of Ag NPs formed in the Tollens-PVAsolution (λmax=422 nm).

Fig. 6. Structure of fully hydrolyzed and partially hydrolyzed PVA.

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PVA is widely used in various industries such as textile, paper, foodpackaging, pharmacy, and cosmetics [151]. Introduction of nanosizedAg into PVA provides antibacterial activity, which is highly desired intextiles used in medicine, clothing and household products [151].However, this can significantly affect the properties of the polymerdue to the high surface to bulk ratio of Ag NPs. [150–153].

Different methods including solvent evaporation, electron radia-tion, UV light, thermal annealing, in situ chemical reduction, andsonochemical have been proposed to synthesize PVA-Ag NPs [154–166]. And a variety of morphologies were obtained under differentpreparation conditions. In the solvent evaporation methods, thesynthesis of PVA-Ag NPs was achieved by first reducing Ag salt withNaBH4, followed by the mechanical dispersion of the Ag colloids intothe dissolved polymer, and then the solvent was evaporated resultingin final structure [154]. The initial average particle size of 5 nm withnarrow size distribution increased to 20 nm with a broad surfaceplasmon absorption band after the dispersion [154]. This particleagglomeration during the incorporation into the PVA matrix resultedin significant changes in the thermal and mechanical properties of thepolymer [154]. The electron and UV radiation preparation methodsinvolve irradiation of a Ag+ doped polymer film which gives PVA-Agcomposites [159–161]. In thermal methods, the annealing time andtemperature vary the morphologies of the PVA-Ag NPs [165,166]. Forinstance, hydrogels of PVA-PVP (poly(N-vinyl pyrolidone) containingAg NPs were prepared by repeated freezing–thawing treatment [167].The hydrogels have unique properties because of their three-dimensional hydrophilic polymer networks, which provide a widerange of pharmaceuticals and medical applications [168]. Using in situchemical reduction, our laboratory had recently synthesized Ag NPs ofcontrolled size by themodified Tollens process with PVA as a stabilizerand reductant [169]. Briefly, one drop of 0.02 M NaOH was added to2 ml of 1 mMAgNO3, followed by 3 drops of 0.2 M NH4OH and 0.25 ml1% aqueous PVA solution. Gentle heating for 2min resulted in a yellowcolored mixture. The visible spectrum showed a maximum at 422 nm,indicating the formation of PVA-Ag NPs (Fig. 7).

4.3. Silver nanoparticles on TiO2

Silver/TiO2 surfaces have advantageous properties such as visiblelight photocatalysis, biological compatibility, and antimicrobial activ-ity [170–176]. Aqueous reduction, photochemical, liquid phasedeposition, and sol–gel methods can be applied to synthesize AgNPs on TiO2 surfaces [177–182]. Ag NPs with a narrow size distributionwere synthesized by simple aqueous reduction from silver ions indifferentmolar ratios of TiO2 suspensions and a reducing agent, NaBH4

[178]. One of the photochemical reduction methods involves loadingAg NPs with ∼3–5 nm diameters onto the surface of TiO2 nanotubesfirst using the liquid deposition approach followed by UV [179] or byfemtosecond laser irradiation [181].

In another photoreduction example, Ag–TiO2 nanocomposites andPVA-capped colloidal Ag–TiO2 nanocomposites have been prepared toinvestigate their antibacterial activity in E. coli and Bacillus subtilis[182]. Photoreduction of AgNO3 at 365 nmwavelength using bare TiO2

and PVA-capped colloidal TiO2 nanoparticles/nanotubes was carried

out. The TEM images of these nanocomposites are given in Fig. 8. Thereactants, TiO2 particles and TiO2 nanotubes, were well dispersed intheir reaction mixtures having a ∼25 nm particle size and a ∼20 nmnanotube diameter with a length of ∼250 nm, respectively (a and b).In the images of the resulting 5 wt.% Ag–TiO2 nanocomposites, the Agon the TiO2 nanoparticles was difficult to visualize (c) yet Ag on theTiO2 nanotubes was clearly evident (d). Next, in-situ PVA-capped TiO2

nanoparticles, ∼20 nm particle size, and PVA-capped TiO2 nanotubeswere prepared (e and f). Photoreduction on these nanocompositesproducts caused Ag aggregation into fairly large colloids. The Ag inPVA-capped Ag–TiO2 particles formed Ag clusters of sizes ∼15 nm (g),while Ag on the PVA-capped Ag–TiO2 nanotubes gave larger, ∼40 nm,Ag particle sizes (h). This work reports that low concentration ofcolloidal Ag–TiO2 nanoparticles and nanotubes were effective indestroying E. coli and B. subtilis [182].

5. Antimicrobial activities

Silver is known for its antimicrobial properties and has been usedfor years in the medical field for antimicrobial applications and evenhas shown to prevent HIV binding to host cells [178,183–186].Additionally, silver has been used in water and air filtration toeliminate microorganisms [187–189].

5.1. Studies: mechanism

The mechanism of the bactericidal effect of silver and Ag NPsremains to be understood. Several studies propose that Ag NPs mayattach to the surface of the cell membrane disturbing permeabilityand respiration functions of the cell [67]. Smaller Ag NPs having thelarge surface area available for interaction would give more bacter-icidal effect than the larger Ag NPs [67]. It is also possible that Ag NPsnot only interact with the surface of membrane, but can also penetrateinside the bacteria [190].

In one study, the Ag NPs obtained in the reduction of the Ag(NH3)2+

complex cation by four saccharides with narrow size distributionweretested as antimicrobial agents (Table 1) [66]. Table 1 shows that Ag NPssynthesized using disaccharides, maltose and lactose, have a higherantibacterial activity than those synthesized using monosaccharides,glucose and galactose. The sizes of the colloidal Ag particles weresmaller for disaccharide than monosaccharide and thus may beresponsible for the observed antibacterial activity. The 25 nm-sized AgNPs synthesized via reduction by maltose (see Table 1) showed thehighest activity and were comparable to the effects of ionic silver incertain bacteria strains. Galactose had the largest Ag NPs particles,50 nm, and gave the lowest antimicrobial effect [66].

Fig. 8. TEM images of (a) TiO2 nanoparticles, (b) TiO2 nanotubes, (c) Ag–TiO2 nanoparticles, (d) Ag–TiO2 nanotubes, (e) PVA-capped TiO2 nanoparticles, (f) PVA-capped TiO2

nanotubes, (g) Ag on PVA-capped TiO2 nanoparticles, and (h) Ag on PVA-capped TiO2 nanotubes (reproduced from [182]) with permission from the American Chemical Society).

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Table 1Minimum inhibition concentrations and minimum bactericidal concentrations of Agparticles prepared via reduction of [Ag(NH3)2]+ by various reducing saccharides at anammonia concentration of 0.005 mol L−1 (data were taken from [66] with permissionfrom the American Chemical Society)

Bacteria Minimum inhibition and bactericidal concentrations(μg/mL)⁎

Saccharide (Ag NP size) Glucose Galactose Maltose Lactose Conta Contb

(44 nm) (50 nm) (25 nm) (35 nm)

Enterococcus faecalisCCM 4224

– – 13.5 54.0 6.75 –

Staphylococcus aureusCCM 3953

6.75 54.0 6.75 6.75 6.75 –

Escherichia coli CCM 3954 27.0 – 3.38 27.0 1.69 –

Pseudomonas aeruginosaCCM 3955

27.0 – 6.75 13.5 0.84 –

Pseudomonas aeruginosa 13.5 27.0 3.38 13.5 0.84 –

Staphylococcus epidermidis1 13.5 6.75 1.69 6.75 0.84 –

Staphylococcus epidermidis2 6.75 54.0 1.69 6.75 1.69 –

Staphylococcus aureus MRSA 27.0 54.0 6.75 27.0 6.75 –

Enterococcus faecium (VRE) – – 13.5 54.0 3.38 –

Klebsiella pneumoniae(ESBL-positive)

27.0 – 6.75 54.0 3.38 –

1 (Methicillin-susceptible); 2 (methicillin-resistant); “–” growth inhibition of bacteriaunsubstantiated.⁎ MIC and MBC of silver sols had same values.

a Control sample containing all initial reaction components without reducingsaccharides.

b Control sample containing all initial reaction components without silver nitrate.

Fig. 9. A plot of minimum inhibition concentration (MIC) of the Ag NPs prepared by themodified Tollens process with D-maltose and consequently modified by addition of SDS,Tween 80, and PVP 360 in concentration of 1% (w/w). (Data were taken from [67] withpermission from the American Chemical Society).

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Enhanced antibacterial activities have been reported in Ag NPsmodified by surfactants, as SDS and Tween 80, and polymers, as PVP360 [67]. The results are presented in Fig. 9. The antibacterial activitywas significantly enhanced for most of the species when Ag NPs weremodified with SDS (Fig. 9). However, the antibacterial effect of Tween80 modified Ag NPs was not significant (Fig. 9). SDS provides morestability to Ag NPs than Tween 80; resulting in a higher antibacterialactivity. Additionally, SDS is an ionic surfactant and may have theability to penetrate or disrupt the cell wall, particularly of gram-positive strains [67,191]. Comparatively, Tween 80 is a non-ionicsurfactant and may not be making contact with the cell wall [67]. Theantibacterial activities of PVP modified Ag NPs were significantbecause the polymer is most effective in stabilizing particles againstaggregation [67].

In another work, the effect of Ag NPs on bacterial growth of E. coli,Vibria cholera, P. aeruginosa, and Syphillis typhus has been studiedusing a high angle annular dark field (HAADF) scanning transmissionelectron microscopy (STEM) technique Fig. 10 [42,190]. Fig. 10 (b) and(c) demonstrate that this technique can identify presence of Ag NPs assmall as ∼1 nm. No significant bacterial growth was observed at AgNPs concentrations above 75 μg/mL. Some noticeable damage to thecell membrane by Ag NPs could be seen (Figs. 10 (b) and 9 (e)). Thedamage to cell may be caused by interaction of Ag NPs withphosphorous- and sulfur-containing compounds such as DNA. Silvertends to have a high affinity for such compounds [192,193]. Ag+ ionsstrongly interact with the available –SH groups of the biomolecule toinactivate the bacteria [194,195]. Furthermore, the antibacterialactivity of Ag+ ion under anaerobic conditions was found less potentthan in oxygen rich environment [195]. Such interactions in the cellmembrane would prevent DNA replications [195,196], which wouldlead to bacterial death [195,197,198].

Involvement of interaction of Ag NPs with bacteria has also beenshown in a study of amine-terminated hyperbranched poly(amidoa-mine) (HPAMAM-NH2)/Ag nanocomposites [199]. The nanocompo-sites had an average particle size of 15–4 nm and the antibacterialactivity was tested against S. typhus, E. coli, B. subtilis, and Klebsiellamobilis [199]. The bacterial activity was inhibited up to 95% by lowconcentrations of the HPAMAM-NH2/Ag nanocomposite, 2.7 μg/mL,and this was comparable to inhibition observed in Ag-doped TiO2, Ag

NPs prepared with surfactant templates, and Ag NPs in a carbonmatrix [195,200,201]. The strong interaction between negativelycharged bacterial wall and HPAMAM-NH2 macromolecules [202–204] can possibly decrease the distance between the Ag NPs andbacteria. This process could facilitate the release of active Ag into thebacteria resulting in a synergistic antibacterial effect of the HPAMAM-NH2/Ag nanocomposites [199].

In proteomic and biochemical studies, nanomolar concentrationsof Ag NPs have killed E. coli cells within minutes possibly due toimmediate dissipation of the proton motive force [205]. This action issimilar to that found for antimicrobial activities of Ag+ ions [206]. Forexample, low concentrations of Ag+ ion result in massive protonleakage through the Vibrio choleraemembrane [206]. This proton leakmight be happening from either any Ag+-modified membrane proteinor any Ag+-modified phospholipids bilayer. The phenomenon causesdeenergization of the membrane and consequently cell death [206].Importantly, the determined effective concentration of Ag NPs was atnanomolar levels while Ag+ ions were effective at micromolar levels[205]. Ag NPs thus seem to be more efficient than Ag+ ions inperforming antimicrobial activities. Picomolar levels of Ag NPs, on theother hand, have been used as nanoprobes in membrane penetrationstudies and did not create significant toxicity to the cells [207].Moreover, the role of Ag+ ion in the antibacterial activity of Ag NPs wasrecently studied by partially oxidizing Ag NPs [208]. The oxygen caneasily oxidize nano-Ag [78,209] to yield partially oxidized nano-Agwith chemisorbed Ag+ ions [208]. The antibacterial activities of Ag NPsagainst E. coli depended on the chemisorbed Ag+ ions (surfaceoxidation) and particle size.

The effect of shape on the antibacterial activity of Ag NPs has onlyrecently been reported [210]. The Ag NPs of different shapes(triangular, spherical, and rod) were tested against E. coli [210]. Thesurfaces of untreated and treated bacterial cells were investigated byenergy-filtering transmission electron microscopy (EFTEM). The {111}facets have high-atom-density, which is favorable for the reactivity of

Fig. 10. (a) bacteria grown on agar plates at different concentrations of Ag NPs. Upper left, E. coli; upper right, S. typhus; bottom left, P. aeruginosa, and bottom right, V. cholerae. 0 μg mL−1

(upper left), 25 μg mL−1 (upper right), 50 μg mL−1 (bottom left) and 75 μg mL−1 (bottom right). HAADF STEM images that show the interaction of the bacteria with the Ag NPs:(b) E. coli, (c) S. typhus, (d) P. aeruginosa, and (e) V. cholerae. The inset correspond to higher magnification images. (reproduced from [190] with permission from the Institute of Physics).

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Ag [211]. A triangular nanoplate has a high percentage of {111} facetswhereas spherical and rod-shaped Ag NPs predominantly have {100}facets along with a small percentage of {111} facets [211].

5.2. The battle against infection: Ag NPs and their incorporation into themedical field

In hospitals, infection is the most common complication and causeof death in patients. Therefore, antibacterial effects of Ag have beenincorporated into various medical applications. Plastic catheterscoated with Ag NPs prevent biofilm formation from E. coli, Entero-

coccus, Staphylococcus aureus, Candida albicans, Staphylococci, andPseudomonas aeruginosa and also show significant in vitro antimicro-bial activity [212]. Silver aerosol NPs were efficient as antimicrobialagents against B. subtilis [213]. Polymethylmetacrylate (PMMA) bonecement loaded with Ag NPs has shown clinical use [183]. Supple-mentation of Ag NPs with antibiotics as penicillin G, amoxicillin,erythromycin, clindamycin, and vancomycine against E. coli and S.aureus has been examined [214]. The presence of Ag NPs increased theantibacterial activities of antibiotics for both strains [214]. Addition-ally, Ag NPs-embedded paints demonstrated killing of both Gram-positive human pathogens and Gram-negative bacteria [215].

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5.2.1. Ag NPs and HIVRecently, a study revealed the potential cytoprotective activity of

Ag NPs toward HIV-1 infected cells [216]. The activity of Ag NPstowards HIV-1 infected Hut/CCR5 cells was investigated usingterminal uridyl-nucleotide end labeling (TUNEL) assay after a threeday treatment [216]. The percentage of aproprotic cells weredetermined as 49%, 35%, and 19% for vehicle control, 5 μM Ag, and50 μM Ag, respectively. Ag NPs might inhibit the replication in Hut/CCR5 cells causing HIV-associated apoptosis [216]. Size dependentinteraction of Ag NPs with HIV-1 virus has also been demonstrated[217]. Ag NPs preferentially binds to gp120 glycoprotein knobs of HIV-1 virus. In the vitro study, it was further shown that this interactioncaused the virus not to bind with the host cell [217].

5.3. Antibacterial water filter

According to the World Health Organization (WHO), point-of-usetreatment has the potential to improve the microbial quality of water

Fig. 11. Antimicrobial effects of the test filters on E. coli (a) and B. subtilis (b). C0 and Care colony count at contact time 0 and t minutes, respectively. (reproduced from [230]with permission from the American Chemical Society).

and reduce the risk of water related diseases such as diarrhea anddehydration [218,219]. Ag NPs on polyurethane foam were stable andwere not washed away by water flow, possibly due to its interactionwith the nitrogen atom of polyurethane [220]. The foam was testedwith an E. coli load of 105 CFUmL−1 at a flow rate of 0.5 L min−1. Withinseconds, the output count of E. coli in the effluent was below thedetection limit [220]. Also, few studies have been conducted on Agcontaining carbon filters for their ability to reduce bactericidal activity[221–223]. Bacteria and fungi were tested and strong lethal activityagainst E. coli, Saccharomyces cerevisiae, and Pichia pastoris wasobserved within a few seconds [221]. The use of reactive oxygenspecies (ROS), a scavenger, and Ag+ ion, a neutralizing agent, suggesteda role of ROS in the strong bactericidal activity of carbon filtersupporting silver [222].

Colloidal-Ag-impregnated ceramic filters were recently tested forhousehold water treatment in the laboratory [224]. The filters removed∼97.8%–100% of the E. coli. Initially, Ag concentrations in the effluentfilter water were greater than 0.1 mg L−1, but decreased to b0.1 mg L−1

after∼200min.Overall, thefindings suggest theuse of the ceramicfiltersas an effective and sustainable point-of-usewater treatment technology.

5.4. Antimicrobial air filter

Bioaerosols are airborne particles of biological origins, which arecapable of causing acute and chronic diseases [225]. Generally,bioaerosols accumulate on ventilating, heating, and air-conditioningsystems with a tendency to multiply under humid conditions [226].Activated carbon fiber (ACF) filters are widely used for removal ofhazardous gaseous pollutants from the air. However, ACF filtersthemselves become a source of bioaerosols because bacteria prefer-entially adhere to carbon solid materials [227,228].

The antimicrobial effect of Ag particles coated onto an ACF filterwas recently tested [229,230]. Ag-deposited ACF filters were effectivefor the removal of bioaerosols. The results of the test filters on E. coliand B. subtilis are shown in Fig. 11. The ACF/Ag-10, ACF/Ag-20, andACF/Ag-30 represent samples prepared with metal solution atdeposition times of 10, 20, and 30 min, respectively without usingelectric current [230]. The colony ratios (C/Co) of both E. coli and B.subtilis increased with time suggesting multiplication of bacteria onthe pristine ACF filters [230]. However, the colony ratios decreased forAg-containing ACF filters; the decrease was sharper for B. subtilis thanfor E. coli species (Fig. 11). Due to low resistivity of B. subtilis, inhibitiontook place in about 10 min [230]. Comparatively, E. coli was fullyinhibited after 60 min using a Ag-containing ACF filters [230].

6. Implications

6.1. Human Health

Nanoparticles may have different effects on human healthrelative to bulk material from which they are produced [15].Increase in biological activity of nanoparticles can be beneficial,detrimental or both. Many nanoparticles are small enough to havean access to skin, lungs, and brain [15,231,232]. Currently, nosufficient information is available on the adverse effects ofnanoparticles on human health [233], but studies are forthcomingto address this subject [234–239]. Exposure of metal-containingnanoparticles to human lung epithelical cells generated reactiveoxygen species, which can lead to oxidative stress and cellulardamage [240,241]. Nanoparticles and reactive oxygen productionhave an established link in vivo [242,243]. A study on toxic effects ofAg NPs was done on zebrafish as a model due to its fast developmentand transparent body structure [244]. The results showed adeposition of particles on organs and severe developmental effects[244]. Similar results of Ag NPs toxicity were observed duringzebrafish embryogenesis [245].

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6.2. Environmental

The increasing use of consumer nanotechnological products mayresult in an increased release of NPs into the aquatic environment[246–248]. Though regulation exists for protecting aquatic speciesfrom soluble forms of toxic metals, it is critical to understand thetoxicity of metallic nanoparticles [249–252]. The studies on the effectof Ag NPs on biological species are forthcoming [253–256]. Asdiscussed previously, proteomic analysis (2-DE and MS identification)was conducted to observe the mode of the antibacterial effect of AgNPs against E. coli [253]. An accumulation of envelope proteinprecursors due to Ag NPs occurred, which suggests the dissipationof proton motive force [253]. Furthermore, the proteomic dataindicate that Ag NPs destabilized the outer membrane, which resultedin a collapse of the plasma membrane potential and depletion ofintracellular ATP levels [253].

Single-NP probes (individual Ag NPs) were developed to study realtime transport, biocompatibility, and toxicity of Ag NPs in the earlydevelopment of zebrafish embryos [254]. It was found that single AgNPs with an average diameter of 11.6±3.5 nmwere transported in andout of embryos through chorion pore canals. The Brownian diffusion,3×10−9 cm2 s−1, inside the chorionic space was determined [254]. Thebiocompatibility and toxicity of Ag NPs were exhibited by observingsingle Ag NPs inside embryos at each development stage. The types ofabnormalities in zebrafish were strongly dependent on the dose of AgNPs [254]. Fish can bioconcentrate trace contaminants in the aquaticenvironment and the potential release of nanomaterials may alsoaffect human health through the consumption of fish.

Furthermore, Ag NPs are of great concern to wastewater treatmentutilities and to biological systems [255]. The inhibitory effects of AgNPs on microbial growth were evaluated at a treatment facility usingan extant respirometry technique [255]. The nitrifying bacteria weresusceptible to inhibition by Ag NPs, which could have detrimentaleffects on the microorganisms in wastewater treatment. The environ-mental risk of Ag NPs was recently investigated by determiningreleased Ag from commercial clothing (socks) [256]. The sockmaterialand wash water contained Ag NPs of 10–500 nm diameter. The fate ofAg inwastewater treatment plants (WWTPs), which could treat a highconcentration of influent Ag, was also examined [256]. The modelsuggested that WWTPs are capable of removing Ag at concentrationsmuch more than expected from the Ag NPs-containing consumerproducts. However, Ag concentrations in the biosolids may exceed theconcentration (5 mg/L), established by the USEPA. This may restrictthe fertilizer application of biosolids to the agricultural lands.

7. Concluding remarks

Several synthetic methods for Ag NPs using inexpensive andnontoxic compounds under water environments were summarizedand experimental approaches under different conditions were givento control the morphology of the Ag particles. Rapid and greensynthetic methods using extracts of bio-organisms have shown a greatpotential in Ag NP synthesis. However, understanding the mechanismby which biomolecules of these organisms are involved in synthesis islacking. A progress in this area will give new green paths in thedevelopment of controlled shape and size Ag NPs. Custom designedbiomolecules can then be made to synthesize Ag NPs, which will inturn fill the gap between biological synthesis and biometric synthesis.Moreover, the syntheses of nanostructures of Ag in high yield and in awide range of shapes are challenging tasks. This requires theunderstanding of the nuclei formation and the influence of reactionspecies on nuclei morphology [24]. The theoretical calculations inconjunction with high-resolution mass spectrometry will help toachieve this objective [257].

Silver incorporated into polymer and TiO2 surfaces have favorableelectronic, photo and catalytic properties. Different synthetic

approaches for the surface modification were provided resulting indifferent particle morphologies. Surfactants and polymers modifiedAg NPs have advantages in antibacterial activities; however theirantibacterial actions are not fully understood. The techniques tomeasure transport of Ag NPs in vivo in real time scales are needed tomake headways in observing particle interactions. Some progress wasmade in a recent study [254] and more such studies should occur inthe future. This will also determine the effect of Ag NPs on importantaquatic species and reveal their environmental consequences.

The increasing use of Ag NPs in consumer products will increasetheir release to the environment and any advancement in nanotech-nology would thus require assessment of environmental risksassociated with these particles [15,258]. The ecotoxic studies on theexposure of Ag NPs need an analytical technique that can distinguishnano-Ag metal from the dissolved Ag+ species under environmentalconditions. Such techniques are becoming more available, but theirapplications at relatively low concentrations are still limited[259,260].

Acknowledgment

We wish to thank three anonymous reviewers for their usefulcomments that greatly improved this paper.

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